On the fifth of December 2022, a group of researchers at the National Ignitions Facility (NIF) in the USA, conducted an experiment which released 3.15 MJ of energy while only using 2.05 MJ of energy [3]. During this experiment, heavy hydrogen atoms were fused into a helium atom, which released energy in the process, this process is called nuclear fusion. This was a historic moment as it marked the first time that more energy was generated by this process than was required to initiate the process, with the only byproduct being Helium. Could this be the breakthrough towards endless clean energy?
Nuclear fusion
With everything we do, we need energy. From growing our foods to sending a satellite into space, none could be done without energy. Practically all energy sources on Earth, in one way or another, come from our Sun. For some sources, this is evident, like solar energy or wind energy that comes from the motion of the air, caused by warming from the Sun. For other sources the relationship is less obvious, take fossil fuels. The energy contained in fossil fuels is solar energy captured through photosynthesis and stored in chemical bonds as plants grow. The energy is then released when we burn it millions of years later once these plants have turned into fossils [1]. If all energy sources on Earth get their energy from the Sun, what powers the Sun?
The core of the Sun is so hot and dense that atoms are stripped of their electrons, such that both nuclei and electrons can move around freely in a so-called plasma. Since nuclei are positively charged, they normally repel each other. To overcome this, the nuclei have to move extremely fast into each other. Since particle speed is just temperature, it means that we need extreme heat for this process. In the Sun’s core, this happens at around 15 million degrees Celsius. However, if you want to do the same on Earth, you will need around 100 million degrees Celsius, due to the difference in pressure between Earth’s atmosphere and the Sun’s core.
In nuclear fusion, two nuclei fuse to create a larger nucleus, this second nucleus will have less mass than the sum of the original two nuclei. We know that mass should always be conserved and in reality, the difference in mass is converted to energy using Einstein’s famous $E=mc^2$ equation. This energy is then released in radiation which is later converted to heat.
The Breakthrough
It was never the question of whether nuclear fusion could exist in our Universe, our Sun is a clear example of that. Also, over sixty years ago, researchers at the Los Alamos National Laboratory were the first to achieve controlled nuclear fusion on Earth. What made this experiment so special was that for the first time ever, the energy released by the experiment was more than the energy put into the experiment. How they achieved this is a very complex and technical story, so I will just give you a brief summary.
At the NIF a pulse of laser light is fired, which gets separated into 192 different laser beam paths. Each of these beams goes through a series of amplification, spatial filtering and polarization steps so that the power is distributed evenly among them and to ensure that all beams arrive simultaneously at the target. The target, or Hohlraum, is a hollow cylinder around the size of a pea. Inside it, you find a capsule containing deuterium and tritium, both being heavy isotopes of hydrogen. The laser beams hit the interior of the Hohlraum, causing it to heat to extreme temperatures, releasing x-rays in the process. These x-rays evenly distribute energy onto the capsule, heating and compressing the light atoms inside, causing these to collide and starting nuclear fusion [2].
As the laser beams hit the target simultaneously, they deliver 2.05 MJ of energy into the target in a few billionths of a second, starting nuclear fusion. On December 5th 2022, this process put out 3.15 MJ of energy, resulting in a net energy gain. In theory, repeating this experiment continuously would lead to an infinite amount of energy. Combine this with the fact that the only waste product of this process is helium and nuclear fusion makes an excellent clean energy source to power our future, but in reality, there are still some hurdles to be taken.
Practical use
If we consider the target to be the complete system, then there is indeed a net energy gain, as explained earlier. However, if we consider the entire NIF facility to be the system, then we also have to account for the energy used for amplification, spatial filtering and polarization steps of the 192 laser beams, increasing the input energy to 300 MJ. However, the equipment used to conduct this experiment is from the ’90s and the laser only has an efficiency of 0.5% whereas modern-day lasers can achieve an efficiency of up to 30% [2]. Using these kinds of lasers would drastically reduce the energy needed for the entire process.
The fuel needed for nuclear fusion is a combination of two different isotopes of hydrogen, deuterium ($^2_1 H$) and tritium ($^3_1 H$). Although hydrogen is the most abundant molecule in our universe, both these isotopes, especially tritium are quite rare. It is expected that at this moment there exist only a few kilograms of tritium on Earth, mostly located in nuclear warheads, making it extremely expensive.
Another problem that arises when scaling up the experiment to a full-on fusion reactor is the discontinuity of the experiment. Currently, the NIF can only conduct roughly one such experiment a day. This is fine for research purposes but makes this experiment setup unviable for a fusion reactor.
One important thing to note about this experiment is that its goal was never to make nuclear fusion economically viable. Its purpose was to prove the concept of net energy gain using nuclear fusion on Earth and to study this process of nuclear fusion. Even though the realization of a fusion reactor generating endless clean energy might still be far away. If humanity ever does realize such a reactor, we might look back upon this experiment as the point where it all started.
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